In general, a mobile communication system was developed for the purpose of providing a voice service while securing user's mobility. However, the area of the mobile communication system has been expanded gradually to data services in addition to voice services; and, at present, the mobile communication system has been developed to the extent of being capable of providing high-speed data services. However, since resources are insufficient in the mobile communication system that is currently providing services and users demand higher-speed services, there is a need for a greatly developed mobile communication system.
To cope with such a need, standardization of a Long Term Evolution (LTE) system in the 3rd Generation Partnership Project (3GPP) is currently underway as one of the next-generation of mobile communication systems that are being developed. The LTE system is a technology to implement high-speed packet-based communication having a maximum transmission speed of about 100 Mbps with the aim of commercialization in about 2010. For this, there have been discussions on several schemes, for example, a scheme to reduce the number of nodes that are positioned on communication paths through simplification of the structure of a network and a scheme to make radio protocols maximally approach radio channels.
Meanwhile, if User Equipment (UE) performs direct communication with an opposite UE without involving an Evolved Node B (ENB), a relatively small amount of radio resources is used in comparison with a case where the communication is performed using an ENB in the existing radio network; thus, radio resource efficiency is greatly improved.
Further, since a method that can find UEs around the UE is supported, the UE can send directly necessary information to a desired UE; thus, efficiency is greatly heightened in supporting an advertisement service or a Social Networking Service (hereinafter referred to as “SNS”).
However, if time synchronization between UEs is not performed during performing of such D2D communication, accurate communication may not be performed, and there is now a need for discussion on this subject.